The present invention relates generally to defibrillators. In particular the present invention relates to a dual cell battery stack configuration for use with a defibrillator.
Cardiac arrest, exposure to high voltage power lines and other trauma to the body can result in ventricular fibrillation which is the rapid and uncoordinated contraction of the myocardium. The use of external defibrillators to restore the heart beat to its normal pace through the application of an electrical shock is a well recognized and important tool in resuscitating patients. External defibrillation is used in emergency settings in which the patient is either unconscious or otherwise unable to communicate.
Automated external defibrillators (AED) are used by first responders such as police officers, paramedics and other emergency medical technicians to resuscitate cardiac arrest patients. The AEDs carried by these technicians must be quickly operational after powering up and must not provide false alarms that might delay rescue. In a high stress situation of cardiac arrest, the technician must be able to rely on the operability of the AED. Studies have shown that the chances of successfully resuscitating the patient decreases approximately ten percent per minute following cardiac arrest.
A defibrillation shock electrical pulse selectively applied to a patient is generated within a defibrillator by high voltage generation circuits with energy stored within a capacitor bank of the defibrillator. The capacitor bank forms part of an electrical system along with the battery pack which provides the energy to be stored in the capacitor banks. These shock pulses carry a considerable amount energy of about 200 to 400 joules. This energy, and the generation of the shock pulse, can be dangerous if not handled properly. Accordingly, maintaining control of this potent electrical force under all conditions is imperative.
A microprocessor is typically used to control a defibrillator and its supporting electrical system for charging and generating the shock pulse. Failure of the microprocessor during charging of a capacitor bank or during application of a shock pulse can be detrimental to the patient since the microprocessor would cease control of the charging or application of the shock pulse. The most basic step in maintaining control of the defibrillator with the microprocessor includes maintaining a reliable supply of power to the microprocessor to insure its operation.
Known defibrillator battery packs have a plurality of battery cells connected in series with multiple sets of cells arranged in parallel. For example, as shown in FIG. 1, a prior art battery pack 5 includes two sets of four battery cells with a first set 7 of battery cells (6A-6D) and a second set 8 of battery cells (6A-6D) connected in parallel. Diodes 9 are arranged at the top of each set of cells. These diodes protect the cells from attempts to charge each other.
A microprocessor of the defibrillation device can be powered by a 5 V supply generated with an external regulator connected to the 12 V cell arrangement. Due to inherent inefficiencies, this method wastes energy and can result in a loss of the 5 V supply when the 12 V supply is lost due to battery depletion or other battery failure. When the voltage of the 12 V cell arrangement fluctuates or dips considerably, the 5 V supply drops below a level sufficient to operate the microprocessor. This voltage drop can cause the microprocessor to malfunction and to no longer control operation of the defibrillation device. Significantly, this lack of control includes no longer controlling the capacitor charging operation already in process. With the microprocessor no longer functioning, the capacitor charging operation continues without regulation by the microprocessor resulting in a charged capacitor bank without safe constraint. Accordingly, a nonfunctioning microprocessor due to a failed battery cell can cause a dangerous condition of having a fully charged defibrillator device with no safety controls or result in misapplication of an ongoing defibrillation shock to a patient.
The use of lithium battery cells in defibrillator battery packs carries additional special considerations. For example, when using lithium sulfur dioxide battery cells, it is critical that the cells not be reversed biased. Reverse biasing can occur if the cells are not properly arranged when connected in parallel and if one attempts to recharge the lithium battery cells in the typical battery pack configuration. If the lithium battery cells become reversed biased, overheating will occur in an irreversible battery cell damaging process. Overheating of the cells causes pressure to build up inside the cells until a violent and noxious outgasing occurs. This battery characteristic limits unrestricted transportation of conventional lithium battery cell packs and the manner of their deployment. Accordingly, lithium battery cells for use in a conventional battery pack configuration require special handling.